In a large heatproof mixing bowl or the bowl of a stand mixer, whisk together (briefly) the sugar, egg whites, and salt., Place the bowl over a saucepan of simmering water (you should just barely see some lazy bubbles coming up), and whisk constantly until the mixture measures 161°F (71.7°C) on a digital thermometer., Remove the bowl from the heat, and beat the meringue with your mixer's whisk attachment until stiff., With the mixer running, add the butter a few tablespoons at a time. Make sure each portion of butter is completely incorporated before adding the next., Once half the butter is in, stop and scrape any sticky residue from the sides and bottom of the bowl. Whisk to combine, then finish adding the rest of the butter., Beat in the flavoring of your choice. (See "tips," below for options.), If the frosting seems soft, chill for 15 minutes before using., Store for up to one week in the refrigerator, or six months in the freezer.